Silicon (Si) has been and still is the center of attention for much work in the world of semiconductors and is still the material of choice for most semiconductor applications. Among the reasons for this are the relative ease of growing c-Si and poly-Si compared to other materials, the relative low number of defects that usually accompany this growth, and finally, the ease of growing SiO2 on Si, which furnishes a dielectric/Si interface with very unique characteristics that make Si far more superior than other semiconductor materials. Recently, Si has gained even more momentum with the advent of epitaxial growth of SiGe and SiC alloys, which may open up the door for Si to emerge as a material of choice for many new applications that were long dominated by the III-V semiconductors.
Amorphous Si (a-Si) has its share of applications and research on a-Si intensified after the arrival of hydrogenated amorphous Si (a-Si:H) in the late 1960s. It was observed that this new material has a much lower defect density than its unhydrogenated counterpart. This was a major improvement in the area of amorphous semiconductor research because it made doping amorphous Si a possibility that was not possible in unhydrogenated silicon. Further research showed that the material had good electrical conductivity with relatively high carrier mobility. In the mid 1970s, reports on n-type and p-type doping of this material were published, which triggered an increased interest in this form of amorphous silicon. Currently, a-Si:H has many commercial applications, for example, in TFTs, the switching elements in most Active Matrix Liquid Crystal Display (AMLCD). Also, a-Si is known to have a pseudo-direct band gap and high absorption coefficient, which makes it suitable for solar cell fabrication.
One of the very attractive features of a-Si:H is that it can be easily deposited at low temperatures as a thin film on large areas. This feature made it a very suitable material in other commercial applications, such as scanners and FAX arrays. It also motivated researchers in the last twenty years or so to come up with methods to crystallize a-Si thin films to obtain low-temperature large area poly-Si thins. Most a-Si:H films are deposited by plasma-enhanced chemical vapor deposition (PECVD). In PECVD, silane gas is introduced into a reaction chamber while a plasma of charged particles is sustained inside by the acceleration of electrons and ions between two electrodes. These accelerated particles collide with the gas molecules causing them to break up into radicals that react at the substrate causing a film of hydrogenated silicon to grow. It is generally known that the hydrogen content of an a-Si:H film increases as the deposition temperature is decreased. The hydrogen content can vary from 40 to 10 at % as the substrate temperature is raised. The defect density in the film is inversely proportional to this content for substrate temperatures above 200–250° C. Also, the defect density increases for substrate temperatures below 200° C. Generally, there are around 1015 cm−3 defect states in an a-Si:H film deposited at 250° C. substrate temperature compared to 1020 cm−3 in non-hydrogenated film.
Polycrystalline Si (poly-Si) has also been an attractive material for many years. It has been used in many electronic applications such as MOS devices where heavily-doped poly-Si can serve as a gate material or for interconnecting. Poly-Si has also been extensively used in fabricating solar cells and, most recently, for thin film transistors (TFTs), which give them higher carrier mobility as compared to amorphous Si-based TFTs. Polysilicon thin film is now attracting a great deal of research and interest in the semiconductor industry because of its superior electronic properties as compared to its amorphous counterpart. Several technologies are available now to produce poly-Si thin films. These technologies are competing to achieve the best quality poly-Si through producing poly-Si with large grain size and high carrier mobility. It is known that the larger the grain size in the poly-Si thin film, the lower the number of defect states in the band gap. The states in the band gap are actually trap states for carriers that are introduced when the long range order in the Si matrix is disrupted. These trap states are present in the grain boundaries. Therefore, reducing the total area of the grain by achieving larger grains reduces the density of the trap states and this enhances the performance of the poly-Si based device significantly.
Solid phase crystallization (SPC) is one method used to convert a-Si to poly-Si. In this method a-Si thin film is deposited at low pressures in a CVD system by thermal decomposition of silane. The substrate is kept at a relatively high temperature of about 600° C. With further heat treatment above 600° C. after deposition, crystallites start to form, which converts the film into poly-Si. The grains increase in size upon heat treatment in furnaces at higher temperatures. The high temperature is needed to help the Si—Si bonds break in the amorphous network and reform, but in a more periodic manner. Larger grains are desirable in thin films. In order to realize a good quality film with large grains, temperatures as high as 1000° C. are sometimes necessary. Using this method, grains as large as 5 μm can be realized. It is still the preferred method to obtain poly-Si films, however its main disadvantage is that it requires high annealing temperatures and annealing times in tens of hours. The high annealing temperatures can be destructive to various device constructs.
A novel method used to produce poly-crystalline silicon thin films is metal-induced crystallization (MIC), which produces poly-silicon thin films by interacting amorphous Si thin films with metals such as Al, Au, Pd, Ag, Ni or Cr. MIC permits crystallizing a-Si at much lower temperatures compared to the temperatures of solid-phase crystallization (SPC) of Si. Temperatures as low as 150° C. have been reported [M. Haque, et al., J. Appl. Phys., 75(8): 3928 (1994)], and metals, such as Al [G. Radnoczi, et al., J. Appl. Phys. 69(9): 6394 (1991)], Ag [A. Robertson, et. al., J. Vac. Tech. A, 5(4): 1447 (1987)], Pd [S. Lee, et al., Appl Phys. Lett., 66(13): 1671 (1995)] and Ni [S. Yoon, et al., J. Appl. Phys., 84(11): 6463 (1998)] are commonly used for this purpose. As far as cost is concerned, MIC is the most economical method for producing good quality poly-silicon (poly-Si) with high throughput.
Results on thin-film transistors (TFTS) fabricated using this method have been reported [S. Lee et al., IEEE Electron Devices Lett., 17: 160 (1996)]. They are interesting and have triggered more research on this subject [M. Wong, et. al., IEEE Electron Devices, 47(5): 1061 (2000); D. Murley, et. al., IEEE Electron Devices, 48(6): 1145 (2001)], and now, complete TFTs can be fabricated by combining vertical MIC and lateral MIC of a-Si. MIC has also been studied for applications in the production of solar cells using Al and other metals to prepare poly-Si seed layers on glass at low temperature [P. Widenborg et al., J. Crystal Growth, 242: 270–282 (2002); P. Widenborg et al., Solar Energy Matls. & Solar Cells, 74: 305–314 (2002)].
MIC represents a challenge to material scientists as far as understanding the mechanism is concerned. While the fast diffusion of Al in a-Si seems to be a reasonable and consistent explanation for low temperature MIC, Si solubility in Al, although finite, can be another explanation. Several models have been proposed to explain this phenomenon for Al [M. Haque, ibid, T. Konno et al., Phil Mag. B, 66(6): 749 (1992); Y. Masaki, et al., J. Appl. Phys., 76(9): 5225 (1994); J. Kim et al., Jpn. J. Appl. Phys., 35: 2052 (1996); M. Ashtikar et al., J. Appl. Phys., 78(2): 913 (1995)] and Ni [S. Yoon, et al., J. Appl. Phys., 84(11): 6463 (1998); E. Guliants et al., J. Appl. Phys., 87(7): 6463 (2000)], and they all seem to agree that the interaction starts at the interface between the Si layer and the metal.
The role that the interface plays in the mechanism of MIC has not been fully studied. Among the first to comment on its role were Kim and Lee [Kim et al., ibid] who studied structures of poly-Al/native SiO2/a-Si. They observed that the crystallization occurred even though a 2 nm thick native oxide was present. The proposed model depends mainly on the mechanism of inter-diffusion. This mechanism involves the diffusion of Si atoms into the Al layer and the diffusion of Al into the Si layer. On the other hand, Nast [O. Nast et al., J. Appl. Phys. 88(2): 716 (2000)] studied the effect of native alumina in layered Al/a-Si/Al structures and observed that the presence of this layer had the strongest effect on the overall crystallization process compared to other parameters. In this paper, the crystallization of samples that have different thicknesses of native SiO2 on their a-Si:H layer was quantitatively compared using x-ray diffraction (XRD), and then, qualitatively using scanning electron microscopy (SEM). The crystallization rates of the samples as a function of the oxide thickness were compared and the crystallinity of the samples was examined by SEM. In both cases it was found that the presence of the native oxide layer strongly affects crystallization.
Several patent references propose methods of promoting crystallization of a-Si using an applied metal layer. Representative of these is U.S. Pat. No. 6,277,714 (issued to Fonash et al.), which proposes inducing crystallization of a-Si by pressing a metal layer into physical contact with the a-Si layer and applying heat. U.S. Pat. No. 6,537,898 (issued to Lee et al.) proposes forming a poly-Si layer using a silicon-metal composite layer in which the metal acts as a catalyst for crystallization. U.S. Pat. Nos. 6,339,013 and 6,613,653 (issued to Naseem et al.) propose low temperature methods of forming semiconductor device, with or without a doping metal, using MIC. U.S. Pat. No. 6,486,496 (issued to Chang et al.) proposes forming poly-Si layers on TFTs using metal-induced lateral crystallization (MILC).
An object of the present invention is the development of new methods and articles of manufacture permitting low temperature crystallization of a-Si. Such methods and devices are expected to have applications to solar cells, transistors, and the like.